This invention relates to optical apparatus for measuring the thickness or chemical composition of partially transparent films, which are either self-supporting or attached to a reflecting subtrate. Such equipment is useful in laboratory settings, and is also highly valuable for process control functions. It is particularly desired to provide an industrial coating monitor which will function on-line during the manufacturing process to measure continuously the thickness of plastic coatings on metal surfaces.
In my previously filed application, Ser. No. 687,240, now U.S. Pat. No. 4,129,781 filed May 17, 1976, I disclosed a film thickness measuring arrangement in which infra-red radiation strikes the film surface at an angle of incidence which is at or near its Brewster's angle. If the radiation is polarized in its plane of incidence, reflection from the film surface is largely eliminated, thereby greatly improving the accuracy of measurement of the film.
In the usual arrangement of this type, the radiation is reflected from a substrate below the film. Detection of the reflected radiation is affected adversely by any failure of the substrate to remain in an unchanging location with respect to the optical apparatus. In other words, target displacement reduces the effectiveness of detection, whether the displacement is due to tilting of the substrate, bending of the substrate, or displacement of the substrate while it remains parallel to its original position.
Prior to development of the apparatus disclosed in my earlier application, the most common arrangement of optical apparatus caused the radiation to strike the film and its substrate on a line perpendicular to their surfaces. While displacement of the substrate in such a situation (radiation normal to the surface) reduces detection efficiency, the off-normal incidence of the radiation on the surface, as in the Brewster's angle design, suffers from increased sensitivity to target displacement, since this shifts the image laterally. As a result, even a small displacement can cause the radiation to completely miss the detector.